In large buildings, such as office buildings, the core of the building is generally isolated from external environmental conditions. As a result, the core of a building is usually cooled year-round due to the heating load of the lights, machinery and personnel while the periphery of the building is heated or cooled, as required. Thus, in such buildings, there is ordinarily a concurrent demand for cooling and heating and/or neutral air to provide temperature regulation and to overcome air stagnation.
Various configurations have been employed to meet the differing demands of different parts of the system. In constant volume systems, a constant delivery fan is used to provide a constant air flow with the character/temperature of the flow being thermostatically controlled. In variable volume systems, many means are used to control fan volume. The fan speed of a variable speed fan can be varied to maintain static pressure requirements while the individually controlled dampers regulate the flow in each zone. Other means of control are riding the fan curve, using inlet guide vanes and using discharge dampers. Additionally, in conventional variable volume systems, only cooled or neutral air is circulated in the system. At locations where heating is required, a local heat source, such as an electric resistance heater, is provided. The air to be heated is provided from a separate source, such as the ceiling plenum, and requires additional fans.
In variable air volume systems where the air flow to each zone is controlled at the conditioning unit, each zone generally has a plurality of air outlets but a single sensor. The single sensor determines the amount of neutral or conditioned air supplied to each zone and is influenced by the cumulative flow through the various air outlets in each zone. The satisfactory operation of such a system requires that the demand required by each air outlet be somewhat uniform. Contrary to this requirement is, for example, a conference room located in a zone and defining a subzone. The infrequent use of such a room, coupled with high attendance when used, would generally find the room in either an overcooled/overheated condition or unsatisfied. An unsatisfied cooling condition would be exacerbated if the occupants of the zone were concentrated in the conference room since demand would be lessened at the zone sensor location. Other areas are corner rooms which have different sun loads, wind exposure etc., than occur in some or all of the other parts of the zone.